Typical gas burner systems include a controller which provides the signals for operating the various elements of the burner system. In a typical such burner system, these elements include a combined pilot/main gas valve receiving gas from an external source and a main only gas valve receiving its flow of gas from the pilot/main gas valve. A pilot burner element receives gas directly from the pilot/main gas valve, and a main burner element receives fuel from the main valve. The burner elements are mounted in a combustion chamber where the gas is burned. An igniter for initiating combustion of the fuel is located directly in the path of gas flow from the pilot burner element.
There are two types of igniters in general use at the present time. One is of the type which generates a spark to cause the ignition. The other type passes current through a resistive element sufficient to heat it to a temperature capable of igniting the gas, and are frequently referred to as hot surface igniters. For reasons of durability and reliability, the hot surface igniter is now usually preferred for most gas burners.
The typical sequence of operation by the controller when heat from such a burner is desired, is first to provide a signal to the igniter which activates it, generating heat and then to provide a signal to the pilot/main valve causing it to open. The fuel flowing to the pilot burner element is ignited by the igniter. As soon as the pilot flame has been established, a sensor detects its presence and provides a flame present signal to the controller. The controller then provides a signal which opens the main valve. The main valve allow gas to flow to the main burner where it is ignited by the flame from the pilot valve. Once the main burner flame is established, it is of course self-sustaining during normal operation. The sensor is important because absence of the flame signal is used by the controller to abort opening of the main valve where the pilot flame has not been established, and to allow the controller to immediately close the main valve if the flame signal vanishes during normal operation. It goes without saying that holding the main and main/pilot valves open when flame is not present creates a very perilous situation. This type of burner controller is described in greater detail in my U.S. Pat. No. 5,035,607 which is assigned to the assignee of this application.
There are a number of different types of flame sensors which can be used in a gas burner installation. One which is used extensively in modern burner installations because it is relatively inexpensive and at the same time extremely reliable is the so-called flame rod sensor. Such a flame sensor is disclosed in the above-mentioned '607 patent. A flame rod sensor relies on the differing areas of a flame rod and the metal pilot burner element to form from them an electrical device which employs the ionized molecules of the flame when present to act as carriers for a current resulting from an AC voltage applied between them. The electrical device thus formed has an impedance from the flame rod to the burner element which is markedly lower than the impedance in the other direction and thus forms a rectifier of sorts. The rectifier connection between the igniter and the burner appears only when flame is present. A simple amplifier with a filtered input can detect the presence of the direct current component of current flow between the flame rod and the burner when flame is present. If this flame rod current flow is at least a preselected flame present level, then flame can be assumed to be present.
Recently, a peculiarity in the operation of flame rod sensors has been noted. On occasion during startup, particularly for flame rods which have been in service for a significant portion of their lifetime, the flame rod signal current will exceed the flame present level for a short period of time, perhaps a second or so, after the pilot flame first appears and then fall for a longer period of time to below the flame present level, even though the pilot flame is fully established. We call this phenomenon the flame rod signal anomaly. The interval immediately following presence of pilot flame where the flame rod signal is above the flame present level and before the flame signal level falls below the flame present level, we call the first anomaly interval. The interval of low flame rod signal current while a bona fide pilot flame exists and which follows a first anomaly interval, is called the second anomaly interval.
The first anomaly interval is typically a second or so long as previously mentioned, and the second anomaly interval may be as long as ten or fifteen seconds. The first anomaly interval is typically long enough to allow the controller to begin the main valve opening phase of the startup sequence. Part way into the main valve opening phase if the anomaly arises, the controller interprets the low flame rod signal current of the second anomaly interval as a pilot flame out condition, and responds by terminating the main valve opening phase and attempting to restart the pilot flame. After the second anomaly interval ends, the flame rod begins to continuously produce a flame rod signal level above the flame present level. The main valve opening phase again starts and proceeds normally to produce normal operation. The anomaly interval does not result in hazardous operation, but it does cause additional actuations of the main and pilot valves and operation of the igniter which may lead to premature failure of these components. The anomaly also creates the impression for someone who is close enough to hear the additional actuations of the valves that the system is operating improperly. Although this is not true, the impression thus created respecting the manufacturer may be adverse.
At the present time we do not know what is the cause of the flame rod signal anomaly, nor do we know how to avoid its occurrence. Nevertheless, it would be advantageous to at least avoid the effects of this anomaly on the operation of the burner system, and thus improve the user's impression of the system's performance.